The invention primarily grew out of needs for making highly reliable, high density dynamic random access memory (DRAM) and other electrical contacts. Advanced semiconductor fabrication is employing increasing vertical circuit integration as designers continue to strive for circuit density maximization. Such typically includes multi-level metallization and interconnect schemes.
Electrical interconnect techniques typically require making electrical connection between metal or other conductive layers, or regions, which are present at different elevations within the substrate. Such interconnecting is typically conducted, in part, by etching a contact opening through insulating material to the lower elevation of a layer or conductive region. The significant increase in density of memory cells and vertical integration places very stringent requirements for contact fabrication technology. The increase in circuit density has resulted in narrower and deeper electrical contact openings between layers within the substrate, something commonly referred to as increasing aspect ratios. Such currently range from 1.5 to 5 and are expected to increase. Adequate contact coverage of electrically conductive materials ultimately placed within these deep and narrow contacts continues to challenge the designer in assuring adequate electrical connection between different elevation areas within the substrate.
As contact openings become narrower and deeper, it becomes more difficult for the artisan to completely fill the contact openings. An example of the problem is best understood with reference to the accompanying FIGS. 1 and 2. There illustrated is a semiconductor wafer fragment 10 comprised of a bulk substrate 12 and an overlying silicon dioxide layer 14, such as borophosphosilicate glass (BPSG). Bulk substrate 12 includes a dopant diffusion region 16 to which electrical connection is to be made. A contact opening 18 is provided through BPSG layer 14 to active area 16.
A thin layer 20 of titanium is deposited atop the wafer to within contact opening 18. Titanium layer 20 is provided to function as a silicide formation layer at the base of contact 18 for reducing resistance. An undesired oxide layer (not shown) also typically forms atop diffusion region 16. The deposited elemental titanium also functions to break-up this undesired oxide and thereafter form a titanium silicide with the silicon of substrate 12 to reduce contact resistance between active area 16 and subsequently deposited plug filling tungsten. Additionally, titanium layer 20 functions as an adhesion/nucleation layer for the subsequently deposited conductive material, for example tungsten. Tungsten does not readily deposit over silicon dioxide and exposed silicon substrate, and the intervening titanium layer 20 facilitates deposition and adhesion of tungsten thereto.
Titanium layer 20 is typically deposited by sputter deposition, and undesirably results in formation of contact projecting cusps 22. This results in a back or re-entrant angle 24 being formed relative to contact opening 18. A layer 26 of tungsten is subsequently deposited with the intent being to completely fill the remaining volume of contact opening 18. Unfortunately, an undesired keyhole 28 typically forms, leaving a void within contact 18.
Referring to FIG. 2, layers 26 and 20 are subsequently etched back by dry etch or chemical-mechanical polishing to form a contact-filling plug 30. Undesirably, this typically opens-up the upper end of keyhole 28. This undesirably creates a thin void which is difficult to clean and rinse during processing. Also in the final construction, the outer surface area of plug 30 is reduced due to the void created by keyhole 28. This counters the desired goal of maximizing electrical contact with plug 30 with a subsequent layer for ultimately making electrical connection with active area 16. Further, the etch back typically conducted to produce plug 30 undesirably over-etches titanium layer 20, forming edge "fangs" 32. Even where a desired overlying metal line and plug filling material constitute the same material deposited in a common step, undesired voids typically form within the contacts.
Prior art techniques have been developed which desirably cause some degree of reflow of the contact filling materials and/or overlying metal conductive lines to facilitate filling of contacts and eliminating voids. One such prior art method subjects the substrate to an extremely high pressure gas phase treatment within a sealed vessel. An example gas phase pressure is around 700 atmospheres and an example temperature of around 400.degree. C. Such conditions apparently cause extrusion of the metal such that it reflows to a slight degree to completely fill contacts, yet without melting to a point of completely losing its previously patterned shape outside of the contacts. One industry process of doing so is referred to as a "force fill" process.
However, such extreme gas pressures and treatment vessels create considerable safety problems to all those working in the vicinity of such vessels. Specifically, if a gas leak or crack were to develop in the reactor vessel, the rapidly expanding gas flowing through such crack could cause the reactor to completely blow apart much like a bomb, or alternately turn the reactor into a lethal projectile.
It would be desirable to overcome these and other problems associated with formation of electrically conductive contact plugs. Although the invention principally arose out of concerns specific to contact filling, the artisan will appreciate that the invention has other applicability in semiconductor processing with the invention only be limited by the accompanying claims appropriately interpreted in accordance with the Doctrine of Equivalents.